Contact lenses prescribed for extended wear can lead to several physiological and morphological changes in the cornea. The standard benchmark of physiological tolerance has been to determine changes in corneal hydration by measuring corneal thickness, the presumption being that the absence of changes in corneal thickness would provide a physiological safe lens. Unfortunately, many of the lenses having high oxygen transmissibility (Dk/L) cause little, if any, change in corneal thickness but may induce certain morphological changes after several months of extended wear. This suggests that either corneal thickness measurements are not sufficiently sensitive to detect small but significant changes in corneal physiology or that other physiological alterations are occurring that may not always be accompanied by a change in corneal hydration. Recently, we have demonstrated that contact lens wear can cause a decrease in stromal pH. There is sufficient biological evidence to indicate that a change in extracellular pH could account for the observed morphological changes accompanying contact lens wear. If the contact-lens-induced decrease in stromal pH is responsible for these morphological changes, then it is important to investigate the effects of stromal pH on corneal integrity. In this study we propose to investigate the range of stromal pH changes accompanying contact lens wear, and then determine the relationship between lens Dk/L, corneal thickness, and development of contact-lens-induced kerotopathies. From these studies, we hope to understand more completely the underlying etiology of corneal changes induced by contact lens wear. determine what lend Dk/L may represent a physiologically safe lens for extended wear, and assess the possibility of using pH measurements as a clinical method for monitoring and predicting corneal changes accompanying contact lens wear and as a test for evaluating corneal health.